Methods and apparatus for treating reactive materials



y 1964 H. F. STERLING 3,140,922

METHODS AND APPARATUS FOR TREATING REACTIVE MATERIALS 2 Sheets-Sheet 1Original Filed Fer'm.v 26,. 1959 I nventor H.F. Sterling Attorney y 1964H. F. STERLING 3,140,922

METHODS AND APPARATUS FOR TREATING REACTIVE MATERIALS Original FiledFeb. 26 1959 2 Sheets-Sheet 2 I nvenlor H-F. Sterling Allorney UnitedStates Patent 3,140,922 METHODS AND APPARATUS FOR TREATING REACTIVEMATERIALS Henley Frank Sterling, London, England, assignor toInternational Standard Electric Corporation, New York, N.Y., acorporation of Delaware Original application Feb. 26, 1959, Ser. No.795,635. Divided and this application Feb. 27, 1961, Ser. No. 92,051Claims priority, application Great Britain Mar. 25, 1958 6 Claims. (Cl.23-277) This invention relates to apparatus for treating reactivematerials.

This application is a division of Serial No. 795,635, filed February 26,1959, now abandoned.

Examples of materials that are reactive when molten or when raised to ahigh temperature are such metals as titanium, zirconium, iron anduranium and some semiconductors of which silicon is a conspicuousexample.

For the melting of titanium or its alloys or Zirconium an arc struckwithin a water cooled copper vessel has been used. An are, however,introduces some contamination from the electrodes into the material.

According to the present invention there is provided a container fortreating reactive materials comprising two coaxial tubes and a memberwhich covers the space between them at one end, the said tubes andmember being made of a metal of high thermal and electricalconductivity, means for circulating cooling fluid through the inner tubeand around the outer tube, and means connecting the inner and outertubes to the terminals of a radio frequency power supply.

The invention will be better understood from the following descriptionof one embodiment thereof taken in conjunction with the accompanyingdrawing.

FIG. 1 shows in cross-section and FIG. 2 in perspective apparatus forthe treatment of silicon and for the production of silicon by thedecomposition of silane.

A container 1 for the material to be treated is constructed with acentral axial tube 2. The container 1 and tube 2 are preferablyconstructed of either copper or silver. The container 1 is surrounded bya jacket 3, for example, of copper and the tube 2 communicates at itslower end with the space between the container 1 and the jacket 3. Aheavy copper plate 4 closes the space between the container 1 and thejacket 3 and is welded to a copper flange 5. A second copper plate 6fits over the plate 4 with the interposition of an electricallyinsulating washer 7 and is welded to a flange 5A. The plates 4 and 6 andthe washer 7 are bolted together by bolts 8 (PEG. 2) which are insulatedfrom both the plates 4 and 6.

The upper plate 6 is furnished with a central aperture 9 over which is aboss 10 welded to the plate 6. A water inlet tube 11 of copper is weldedto the central axial tube 2 and passes through the boss 10 with a vacuumseal 12. The seal 12 is composed of electrically insulating material.Apertures are also provided in plate 6 for an inspection tube 13 and foranother tube 14 the purpose of which will be described hereinafter. Theupper end of the inspection tube 13 is closed by a glass plate 15secured to the end of the tube by a vacuum tight seal.

The ends of tubes 13 and 14 are surrounded by a water jacket 16 throughwhich water may be circulated by means of tubes 17 and 18 (FIG. 2).

Water for keeping the walls of the container 1 cooled is circulated inthrough the tube 11 and out by a tube 19 (not shown in FIG. 2). A tube20 passes through the outer jacket 3 and opens into the wall of thecontainer 1 tangentially thereof.

The flanges 5 and 5A of the plates 4 and 6 are connected to theterminals of the secondary of a transformer 21, FIG. 2, the primary ofwhich is supplied with radio frequency energy from a suitable source.The flange 5 transmits the energy from the terminal to which it isconnected to the outer wall 1 of the crucible. The flange 5a transmitsthe energy from the other terminal via plate 6, the boss 10, the heavycopper strapping 22 which conmeets the boss 10 to the exterior of tube11, and finally via tube 11 to the inner crucible wall 2. High frequencyenergy is thus supplied between the central tube 2 and outer coaxialwall 1 which thus constitute inner and outer conductors of a coaxialtransmission line, short circuited at one end, for the high frequencyenergy supplied. The high frequency field in this transmission line isutilised for the treatment of reactive materials.

As an example, the use of the above apparatus for the decomposition ofsilane to silicon will be described. For this purpose it is necessary toplace a preliminary charge of silicon within the furnace and this chargemust be sufiiciently preheated to lower the resistance thereof to apoint at which eddy currents can be induced therein by the highfrequency field. For the purpose of introducing this charge into thefurnace, the upper end of tube 14 is joined by a vacuum tight joint 23to a charging tube 24, which may be of high purity silica. In a sidebranch 25 of tube 24 is a plunger 26 which protrudes into the tube 24and holds a slug 27 of silicon in the upper portion of tube 24. Aresistance coil 28 (not shown in FIG. 2) is placed over the upperportion of tube 24 to surround the slug 27. Coil 28 is supplied withsuitable current to heat it and thereby heat the slug 27 by radiantheat. When the slug 27 has been heated to the temperature of dull redheat the plunger 26 is operated, for example, by an external magnet andthe slug 27 drops into the furnace. Here it is melted by the eddycurrents induced therein.

Silane is then passed in through the tube 20. The silane is decomposedin the furnace to silicon and hydrogen. The silicon is deposited on tothe pool of molten silicon already present and is melted while thehydrogen and any undecomposed silane is drawn out through an outlet tube29.

The conditions in which decomposition of silane to silicon is efiectedmay vary over a wide range. For example, the silane may be diluted withan inert gas such as argon to the same molecular concentrations (lessthan normal) as described in British Patent No. 745,698. To assist inchecking the required degree of vacuum (or pres sure) within thefurnace, the sight tube 13 is provided with a side connection 30 for amanometer (not shown).

On a restricted surface it is possible to increase the concentration andpressure of the silane which is decomposed provided the temperaturewithin the furnace is maintained above 600 C. By means of this inventionit is thus possible to obtain a higher rate of production of very puresilicon than in the process according to the above mentioned patent.

It can be seen that using my apparatus and method no contamination ofthe silicon from the walls of the furnace will occur. Although theapparatus has been described above for the decomposition of silane tosilicon it is equally useful for any treatment of other highly reactivematerials such as titanium, zirconium, iron and uranium, which treatmentinvolves melting of the substance. The apparatus may also be used forthe production of other semi-conductors such as germanium and boron bydecomposition of a gaseous compound thereof in a manner similar to thatused for the decomposition of silane to silicon.

The process may become a continuous one by providing an overflow tube31, FIG. 1, sealed into the container 1 in a vacuum tight manner so thatwhen molten material accumlates in the container 1 to the height of tothe tube 31 it overflows down this tube into a suitable container.

It is clear also that although the coaxial furnace has been shown asplaced vertically it could equally well be arranged horizontally or atany desired angle to the horizontal.

What I claim is:

-1. A container for treating reactive materials comprising two coaxialtubes and a member which covers the space between them at one end, thesaid tubes and member forming the crucible in which the material istreated, the said tubes and member being made of a metal of high thermaland electrical conductivity, means for circulating cooling fluid throughthe inner tube and around the outer tube, and means connecting the innerand outer tubes to the terminals of a radio frequency power supply thusutilizing the tubes themselves as coaxial conductors of the radiofrequency supply.

2. A container according to claim 1 in which the connecting meanscomprises two disc-like members separated 20 3. A container according toclaim 2 which further comprises a projecting tube and means connected tothe said projecting tube whereby material may be preheated before beingintroduced into the said container.

4. A container according to claim 3 and further comprising means forintroducing gaseous compounds into the said container.

5. A container according to claim 4 and further comprising means forremoving excess material from the said container.

6. A container according to claim 5 and further comprising means forinspecting the inside of the said container during the treatment of thesaid reactive materials.

References Cited in the file of this patent UNITED STATES PATENTS2,013,652 Hall Sept. 10, 1935 2,828,201 Findlay Mar. 25, 1958 2,935,386Selker May 3, 1960 FOREIGN PATENTS 546,346 Canada Sept. 17, 1957

1. A CONTAINER FOR TREATING REACTIVE MATERIALS COMPRISING TWO COAZIALTUBES AND A MEMBER WHICH COVERS THE SPACE BETWEEN THEM AT ONE END, THESAID TUBES AND MEMBER FORMING THE CRUCIBLE IN WHICH THE MATERIAL ISTREATED, THE SAID TUBES AND MEMBER BEING MADE OF A METAL OF HIGH THERMALAND ELECTRICAL CONDUCTIVITY, MEANS FOR CIRCULATING COOLING FLUID THROUGHTHE INNER TUBE AND AROUND THE OUTER TUBE, AND MEANS CONNECTING THE INNERAND OUTER TUBES TO THE TERMINALS OF A RADIO FREQUENCY POWER SUPPLY THUSUTILIZING THE TUBES THEMSELVES AS COAZIAL CONDUCTORS OF THE RADIOFREQUENCY SUPPLY.